Enhancement of carrier (electron and hole) mobility is an important factor for high performance of a transistor. Since an impurity present in a channel causes deterioration of the carrier mobility, it is necessary to form a channel region while preventing impurity diffusion into a surface of a silicon substrate. It is thus well known that a channel structure having a steep impurity concentration gradient is desirable to improve transistor characteristics.
Accordingly, there is a method in which a channel structure having a steep impurity concentration gradient is formed by forming a non-doped silicon epitaxial layer after forming an impurity channel layer by ion implantation.
In this structure, an impurity in the impurity channel layer is diffused from the impurity channel layer into the non-doped silicon epitaxial layer, which causes that a channel profile is moderated. Therefore, since a SiC layer suppresses the diffusion of the impurity (e.g., disclosed in JP-A-2000-77654), it is suggested that, after forming an impurity channel layer by ion implantation, an SiC layer is epitaxially grown on the impurity channel layer and a non-doped silicon epitaxial layer is formed thereon (e.g., disclosed in a non-patent literary document of T. Ernst et al. “2003 Symposium on VLSI Technology Digest of Technical Papers” pp. 51-52).
However, in this structure, there is a problem in that junction capacitance and junction leakage are increased since an impurity diffuses downwards from the impurity channel layer and an impurity concentration at an interface between a well region and a high concentration diffusion layer region and an interface between a channel region and the high concentration diffusion layer region is increased.